Even a single-engine ducted-fan, VTOL aircraft can deliver modest payloads by air, remotely and inexpensively. For example, the payload may be a task-performing arm for cleaning high-voltage insulators that are located in positions high above ground level and are difficult to get at. The payload may be a data acquisition package for use in such environments as prisons or along a military front. For example, these ducted-fan VTOL aircraft may help guards in prison security situations, or may serve to seek out and even destroy military tanks, for tanks can be destroyed with very light munitions. Such VTOL devices may also be used to clean or inspect apparatus in high, remote, and/or dangerous areas, and for data acquisition, even in hostile environments. They may be used for remote painting or for traffic surveillance. They can perform such simple tasks as routine inspection and simple cleaning. They may be used in cattle range operations.
These, of course, are only examples of where such aircraft may be used. These aircraft need not carry a person, although, in large embodiments, that becomes possible. They may be controlled entirely robotically or by remote control via electrical cables or by radio.
The main difficulties with such apparatus heretofore have been to obtain precise control and to do so in a relatively inexpensive manner that is easy for an operator to employ.
Heretofore, most of the efforts to control such vehicles have relied on the tilting of vanes in the slip stream. Depending on their position or deflection, such vanes have been able to provide a moderate degree of control. For example, a vane well below the center of gravity tended to rotate the vehicle about its center of gravity, thereby shifting the vertical lift vector away from the gravitational direction and creating a lateral thrust component enabling movement of the vehicle in the direction of the tilt of the lifting vector. However, in this instance a secondary counteracting effect was also generated, because the vane forces were at right angles to the vane's chord and in the opposite direction from the desired direction of motion; as a result, this counteraction reduced the effectiveness of the principal action as a control means. When the vanes were mounted closer to the center of gravity, this counter force became even greater and tended to equal the force generated by the rotating lift vector, so that there was no control power whatever.
Because of this fact, such vanes have generally been mounted quite far behind the propeller or fan, thereby requiring a long duct or resulting in a system that had a reduced translational speed due to its increased drag. Experience has shown that such type of control was only marginally effective, except at low speeds and in calm conditions. In a crosswind, parasitic drag, due to the components below the center of gravity, tended to make the aircraft difficult to orient and to control.
An additional factor that hampered control of single engine ducted fan type of aircraft was that there were gyroscopic moments due to the rotating fan and to its engine components. If control was to be obtained by tilting the vehicle, then these gyroscopic forces tended to interfere with the vehicle's action. These gyroscopic forces were very time-dependent and prevented control of a tilting aircraft when the required rate of correction of the tilt was high, as, for example, in turbulent air. While counterrotating fans might have helped, these would have been expensive, heavy, and inefficient.
An alternative control means used spoilers in the airstream. For example, if one desired to move the VTOL aircraft to the left, spoilers on the left side of the duct would be employed in the airstream and would reduce the thrust on that side. The left side would then drop, so that the vehicle would tilt and would then translate to the left. While this was a more positive type of control, it had two serious negative effects:
(1) it tended to reduce significantly the overall lift capability of the vehicle, especially if a modest to high translational speed was needed, or if station holding was required in even a modest crosswind; PA1 (2) there was a sizable coupling between the pitch-and-roll axis and the vertical or heave direction. In other words, as the spoilers were engaged and disengaged, the vehicle fell and rose. PA1 (1) The forces generated by swinging a rigid vane are highly nonlinear relative to the changing angle of the vane, and particularly when the aircraft is near the stall condition. PA1 (2) The stall condition is reached by rigid vanes at fairly low angles of vane deflection, generally less than 15.degree.. However, for significant translational forces, such as those which are required to move a vehicle of this type at a velocity greater than one-third of the slip stream velocity, the slip stream deflection required becomes significant and is greater than 15.degree.. It is very difficult, if not impossible, to achieve such deflection with a rotating rigid vane without stalling the vane.
Also, in spoiler types of control, gyroscopic moments continued to present a problem.
Significant spoiling of the airflow also adversely affected the efficiency of the fan and increased the noise generated by the fan.
Other proposed control methods include differential control of the fan blade angles. While this might be effective and efficient, it would also be heavy, reducing the pay load, and very expensive. This technique is employed by helicopters, where it is appropriate. With small single-duct aircraft, the gyroscopic moments would increase with systems employing vane or spoiler controls, and, in fact, would become prohibitively heavy and expensive in connection with pitch control.
Extensive testing of both deflection-vane and spoiler systems and combinations of both, has resulted in the conclusion that single-engine designs requiring the vehicle to tilt, to generate control power, or to provide translation, were not practical except in ideal environments and at low translational speeds.